Page 16
GENERAL ANTENNA RULES
The following general rules should help in maximizing antenna performance:
1.
Proximity to objects such as a user’s hand or body, or metal objects will
cause an antenna to detune. For this reason the antenna shaft and tip
should be positioned as far away from such objects as possible.
2.
Optimum performance will be obtained from a 1/4- or 1/2-wave straight whip
mounted at a right angle to the ground-plane. In many cases this isn’t
desirable for practical or ergonomic reasons; thus, an alternative antenna
style such as a helical, loop, patch, or base-loaded whip may be utilized.
3.
If an internal antenna is to be used, keep it away from other metal
components, particularly large items like transformers, batteries, and PCB
tracks and ground-planes. In many cases, the space around the antenna is
as important as the antenna itself.
4.
In many antenna designs, particularly 1/4-wave whips, the ground-plane
acts as a counterpoise, forming, in essence, a 1/2-wave dipole. For this
reason adequate ground-plane area is essential. As a general rule the
ground-plane to be used as counterpoise should have a surface area
≥
the
overall length of the 1/4-wave radiating element.
5.
Remove the antenna as far as possible from potential interference sources.
Any frequency of sufficient amplitude to enter the receiver’s front end will
reduce system range, cause increased bit errors, and can even prevent
reception entirely. There are many possible sources of internally generated
interference. Switching power supplies, oscillators, even relays can also be
significant sources of potential interference. Here again, the single best
weapon against such problems is attention to placement and layout. Filter
the module’s power supply with a high-frequency bypass capacitor. Place
adequate ground-plane under all potential sources of noise. Shield noisy
board areas whenever practical.
6.
In some applications it is advantageous to place the receiver and its
antenna away from the main equipment. This avoids interference problems
and allows the antenna to be oriented for optimum RF performance. Always
use 50
Ω
coax such as RG-174 for the remote feed.
Antenna resonance should not be confused with antenna impedance. The
difference between resonance and impedance is most easily understood by
considering the value of VSWR at its lowest point. The lowest point of VSWR
indicates the antenna is resonant, but the value of that low point is determined
by the quality of the match between the antenna, the transmission line, and the
device it is attached to.
To fully appreciate the importance of an antenna that is both resonant and
matched consider that an antenna with a VSWR of 1.5 will effectively transmit
approximately 95% of its power while an antenna with a VSWR of 10 will only
transmit about 30%. (See pg. 23)
